Gas separation by membranes is known and comprises a very efficient chemical process technology. For example, there is currently substantial commercial interest in the use of semipermeable membranes for the removal of gases regarded to be impurities from hydrocarbon gases, such as the acid gases H.sub.2 S and CO.sub.2 from natural gas, coal gases, landfill gases and refinery gases. Other examples include the separation and recovery of hydrogen and hydrocarbons in refineries and the recovery of oxygen-enriched and nitrogen-enriched air.
Notwithstanding such recognized interest in gas separation by semipermeable membranes, owing to the state of the art of such membranes, namely, their relatively low flux as to desired gas species and relatively great thickness, the capital cost of a membrane gas separation plant currently tends to be prohibitive. There is therefore a great need in the art for efficient, effectively thin, semipermeable membranes that are inexpensive to make.
A number of techniques for the manufacture of such membranes are known. Asymmetric memhranes having a very thin permselective "skin" layer atop a much thicker microporous sublayer have been prepared from polysulfone and poly(2,6-dimethyl)phenylene oxide by a modified Loeb-Sourirajan phase inversion technique, as disclosed in U.S. Pat. No. 3,709,774. However, there are two principal problems with such membranes: (1) it is often difficult to control the casting process to produce the desired membrane morphology--namely, a membrane with a skin that is non-porous and yet thin enough to be useful; and (2) phase-inversion solvent systems and casting procedures have not been identified for many of the membrane polymers of interest. The first problem--that the nominally dense skin layers of assymmetric membranes typically exhibit surface porosities in the parts-per-million range that compromise selectivity--can be dealt with for at least certain types of asymmetric membranes such as polysulfone by physically coating them with high-permeability elastomers that serve to fill surface pores and defects. See Henis et al., 8 J. Membrane Sci. 233 (1981) and U.S. Pat. No. 4,203,463. In this fashion, the intrinsic gas permeability properties of the so-coated asymmetric substrate may be enhanced. But this improvement is at the same time an inherent limitation in that the membrane's gas permeability properties are derived primarily from the substrate. Even with this improvement, the second limitation of asymmetric membranes mentioned above remains: the number of polymers from which asymmetric membranes can be made is limited.
One approach for obtaining asymmetric skinned membranes with improved gas permeation properties involves treating fabricated membranes, while under onesided constraint, with a swelling agent or solvent mixture so as to cause a collapse of the small pores of the top skin of the asymmetric membrane. See U.S. Pat. No. 4,527,999. However, such a technique is not commercially feasible since the process conditions are too critical.
Another method for preparing ultrathin films involves the creation of a two-layer structure in which a thin and selective film is supported by a microporous backing. The backing is made sufficiently porous so as to minimize its flow resistance, while its pores are made sufficiently small that the burst strength of the thin membrane is not exceeded where it spans the mouth of a pore. Perhaps the simplest method for preparing coated films involves casting a thin layer of polymer solution directly atop a microporous support and then allowing the solvent to evaporate, as described in U.S. Pat. No. 4,134,743 and 9 J. Membrane Sci. 121 (1981). This technique suffers from several drawbacks, the most serious of which is the matter of controlling the coating and penetration processes with sufficient precision so as to guarantee the production of uniformly thin yet defect-free coatings. U.S. Pat. No. 4,243,701 discloses a variation of this procedure wherein one surface of a microporous support membrane is dip-coated with a homogeneous solution containing a prepolymer and a crosslinking agent; the latter are subsequently reacted at elevated temperature. Silicone coatings as thin as a few microns may be prepared in this manner.
Still other efforts at fabricating a semipermeable thin-film composite membrane utilize reverse osmosis (RO)-type technology such as water- or fluid-casting. The primary disadvantage of water casting as applied to the production of ultrathin gas-separation membranes is its cost. Water casting is a delicate, multi-step, and labor-intensive procedure that demands considerable skill and dexterity on the part of the technician and that is best performed under clean-room conditions. As a result, it has been applied commercially only on a relatively small scale, e.g., in the manufacture of membrane oxygen enrichers for home medical oxygen therapy. See U.S. Pat. No. 4,174,955.
None of the foregoing techniques lends itself to the production of efficient, inexpensive membranes needed to reduce the capital cost of a large scale membrane gas separation plant.
The preparation of RO desalination membranes by interfacial polymerization (IP) is known. See Cadotte et al., "Thin-Film Composite Reverse-Osmosis Membranes: Origin, Development, and Recent Advances," in Vol. I of Synthetic Membranes, ACS Symposium Series 153 (1981). Such membranes comprise an ultrathin layer of permselective polymer supported by a microporous membrane made of a different material. The ultrathin separating layer of the thin-film composite (TFC) used in RO is on the order of several hundred to several thousand angstrom units thick, and because of this, TFC membranes exhibit high water fluxes in RO applications. In interfacial polymerization, a microporous support membrane (typically, polysulfone) is first impregnated with an aqueous solution of a multifunctional reagent (e.g., a polyamine), and then immersed in an immiscible solvent containing a crosslinking agent. Polymerization occurs rapidly at the solution interface to form the thin film that serves as the permselective layer. The continuous film that is formed acts as a barrier that retards further contact of the reacting species. This self-limiting process accounts for the excedingly thin permselective films that are obtainable.
IP techniques have recently been applied to the development of TFC gas separation membranes. See U.S. Pat. No. 4,493,714 which discloses composite membranes especially useful for separating oxygen and nitrogen, the membrane being formed by interfacial polymerization of a silicon-containing polyamine and a polyisocyanate or a polyamine and a silicon-containing polyisocyanate on a microporous support to form a silicon-containing ultrathin polyurea film on the support. However, this patent does not disclose membranes useful for other gas separations of interest, such as carbon dioxide and hydrogen sulfide from methane and low molecular weight (C.sub.2 -C.sub.4) hydrocarbons and does not disclose any polycondensation membranes other than polyureas.